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If a person’s genetic code were written into a standard-size book, that book would be a twenty-story tome consisting of three billion permutations of the letters A, C, G, and T, which represent the four nucleobases—adenine, cytosine, guanine, and thymine. The specific arrangement of these four nucleobases creates the code for all biological life on the planet. This code is the genotype, and the way it physically expresses in a life-form (eye color, for instance), combined with its interactions with the environment, is called the phenotype. But understanding the correlation between genotype and phenotype—which DNA code programs which traits—still largely eludes us.
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